1. Field of the Invention
The present invention relates to a fuel cell formed by stacking an electrolyte electrode assembly and separators alternately. The electrolyte electrode assembly includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode.
2. Description of the Related Art
For example, a solid polymer fuel cell employs a polymer ion exchange membrane as a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly. Each of the anode and the cathode is made of electrode catalyst and porous carbon. The membrane electrode assembly is sandwiched between separators (bipolar plates) to form the fuel cell. In use, generally, a predetermined number of the fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas (reactant gas) such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy. An oxidizing gas (reactant gas) such as a gas chiefly containing oxygen (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
In the fuel cell, a fuel gas flow field is formed on the separator facing the anode for supplying the fuel gas to the anode. An oxygen-containing gas flow field is formed on the separator facing the cathode for supplying the oxygen-containing gas to the cathode. Further, a coolant flow field is provided between the anode side separator and the cathode side separator such that a coolant flows along the surfaces of the separators.
Normally, the separators of this type are formed of carbon material. However, it has been found that it is not possible to produce a thin separator using the carbon material due to factors such as the strength. Therefore, recently, attempts to reduce the overall size and weight of the fuel cell using a separator formed of a thin metal plate (hereinafter also referred as the metal separator) have been made. In comparison with the carbon separator, the metal separator has the higher strength, and it is possible to produce a thin metal separator easily. The desired reactant flow field can be formed on the metal separator by pressure forming to achieve the reduction in thickness of the metal separator.
For example, a fuel cell 1 shown in FIG. 18 includes a membrane electrode assembly 5 and a pair of metal separators 6a, 6b sandwiching the membrane electrode assembly 5. The membrane electrode assembly 5 includes an anode 2, a cathode 3, and an electrolyte membrane 4 interposed between the anode 2 and the cathode 3.
The metal separator 6a has a fuel gas flow field 7a for supplying a fuel gas such as a hydrogen-containing gas on its surface facing the anode 2. The metal separator 6b has an oxygen-containing gas flow field 7b for supplying an oxygen-containing gas such as the air on its surface facing the cathode 3. The metal separators 6a, 6b have planar regions 8a, 8b in contact with the anode 2 and the cathode 3. Further, coolant flow fields 9a, 9b as passages of a coolant is formed on back surfaces (surfaces opposite to the contact surfaces) of the planar regions 8a, 8b. 
However, in the metal separators 6a, 6b, the shapes of the coolant flow fields 9a, 9b are determined inevitably based on the shapes of the fuel gas flow field 7a and the oxygen-containing gas flow field 7b. In particular, in an attempt to achieve the long grooves, assuming that the fuel gas flow field 7a and the oxygen-containing gas flow field 7b comprise serpentine flow grooves, the shapes of the coolant flow fields 9a, 9b are significantly constrained. Therefore, the flow rate of the coolant in the electrode surface is not uniform.
Thus, the coolant is stagnant in some area of the coolant flow fields 9a, 9b of the metal separator 6a, 6b, and the coolant may not flow uniformly over the entire surfaces of the metal separators 6a, 6b. Therefore, it is difficult to cool the electrode surfaces uniformly to obtain the stable power generation performance.
In view of the above, for example, Japanese Laid-Open Patent Publication 2002-75395 discloses a separator of a fuel cell. The separator is a metal separator, and includes two corrugated metal plates having gas flow fields, and a corrugated metal intermediate plate sandwiched between the two metal plates. The metal intermediate plate has coolant water flow fields on both surfaces.
However, according to the conventional technique, the metal separator has three metal plates including the two metal plates having gas flow fields, and the one intermediate metal plate having the coolant flow fields on its both surfaces. Therefore, in particular, when a large number of metal separators are stacked to form the fuel cell stack, the number of components of the fuel cell stack is large to increase the production cost, and the dimension in the stacking direction of the metal separators is large. Thus, the overall size of the fuel cell stack is large.